Integrated circuits are interconnected networks of electrical components fabricated on a common foundation called a substrate. The electrical components are typically fabricated on a wafer of semiconductor material that serves as a substrate. Various fabrication techniques, such as layering, doping, masking, and etching, are used to build millions of resistors, transistors, and other electrical components on the wafer. The components are then wired together, (i.e., interconnected) to define a specific electrical circuit, such as a processor or a memory device.
There is a general desire to reduce the sizes of the various components in integrated circuit fabrication. Reducing size is generally accompanied by a reduction in cost, as more and more devices can be fabricated on a single substrate, and a reduction in power requirements, as less power is needed to switch smaller components. However, this size reduction does not come without a cost. As integrated circuit devices become smaller and smaller, charge leakage and parasitic capacitance between components become increasingly problematic. An example of the detrimental impact of charge leakage and parasitic capacitance can be seen in Flash memory devices.
Flash memory devices are one particular class of memory devices that have developed into a popular source of non-volatile memory for a wide range of electronic applications. Non-volatile memory is memory that can retain its data for an extended period without the application of power. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of a charge storage node, such as a floating gate, trapping layers, or other physical phenomena, determine the data state of each cell. By defining two or more ranges of threshold voltages to each correspond to individual data states, one or more bits of information may be stored in each cell. Common uses for Flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules. The uses for non-volatile memory continue to expand to cover more devices and larger amounts of memory.
For a Flash memory device utilizing floating-gate memory cells, where a level of charge stored on the floating-gate affects its threshold voltage, a reduction in size means less volume for charge storage. If the same material were used for the floating gates of two differently sized memory cells, the smaller memory cell would be capable of storing less charge. As a result, the smaller memory cell will have a smaller difference in its possible threshold voltages relative to the larger memory cell. Furthermore, any charge leakage, such as stress-induced gate leakage, would have a larger impact on the threshold voltage of the smaller memory cell. In addition, due to parasitic capacitive coupling to floating gates of adjacent memory cells, more margin may be required to avoid a false reading of the data state of the memory cell. Compensating for leakage and parasitic capacitance concerns with a smaller range of threshold voltages makes it increasingly difficult to distinguish between differing data states of smaller memory cells.
The inventors have appreciated that for the reasons stated above, and for other reasons that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative structures and their processes in the formation of integrated circuit devices.